do you mean track automation ?
I guess you want to know about the different "modes" : manual, play, write and touch.

If you have a track, click on the "a" button of the track header, select which kind of automation you want to edit and make sure you are in the "object mode" (type O in the main track canvas). The automation track should come up. This is where you will fiddle around with the automation curve.

The modes I use practically all the time are manual and play. In manual, you can create a curve manually, point by point by left-clicking in the automation track. But it won't play the automation curve during active transport. For that, you need to go into "play" mode. You can stay in play mode and adjust the curve on the fly too by dragging a point (left-click an existing point + mouse drag), creating (left-click to the location you want), deleting (shift + right-click on point to be deleted) while the main transport is active..

There are mouse / keyboard tricks to manage the automation of a group of selected points. Some are intuitive, some you have to guess a bit or read some ... ahem ... manual :)

I believe, the "write" mode is to record a curve on the fly by playing with the gain fader in realtime. So say you have a control surface device, with a fader button connected to the current track fader slider. In automation write mode, you can play around with the fader of your control surface and see the curve being written in real time.

All this is valid for other automation types (e.g. plugin parameters, or panning).

automation can be used for different reasons, both aesthetic and corrective. You may have uneven recordings that you want to even up in terms of gain. You can also apply panning automation if you wish to have some audio going from one stereo end to the other, etc.

The whole point is to have all these effects repeatable every time you activate transport.

You can of course use automation curves on the master track, nothing prevents you from doing so, the master track is just another bus, it is just the end sink of your multitrack mix. But that's also why you want to treat it with care :)